Gfilui

How to solve this,

root@work:/# sudo apt-get update
Hit:1 http://ppa.launchpad.net/nilarimogard/webupd8/ubuntu xenial InRelease
Get:2 http://security.ubuntu.com/ubuntu xenial-security InRelease [102 kB]
Hit:3 http://nyc2.mirrors.digitalocean.com/ubuntu xenial InRelease
Hit:4 http://nyc2.mirrors.digitalocean.com/ubuntu xenial-updates InRelease
Hit:6 http://nyc2.mirrors.digitalocean.com/ubuntu xenial-backports InRelease
Get:5 http://ag-projects.com/ubuntu xenial InRelease [7,109 B]
Err:5 http://ag-projects.com/ubuntu xenial InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F74046C316D8F9F5
Fetched 109 kB in 0s (181 kB/s)
Reading package lists... Done
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://ag-projects.com/ubuntu xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F74046C316D8F9F5
W: Failed to fetch http://ag-projects.com/ubuntu/dists/xenial/InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F74046C316D8F9F5
W: Some index files failed to download. They have been ignored, or old ones used instead.
*****************************

root@work:/# sudo apt-key list
/etc/apt/trusted.gpg
--------------------
pub 1024D/437D05B5 2004-09-12
uid Ubuntu Archive Automatic Signing Key <ftpmaster@ubuntu.com>
sub 2048g/79164387 2004-09-12
pub 4096R/C0B21F32 2012-05-11
uid Ubuntu Archive Automatic Signing Key (2012) <ftpmaster@ubuntu.com>
pub 4096R/EFE21092 2012-05-11
uid Ubuntu CD Image Automatic Signing Key (2012) <cdimage@ubuntu.com>
pub 1024D/FBB75451 2004-12-30
uid Ubuntu CD Image Automatic Signing Key <cdimage@ubuntu.com>
pub 1024D/163A0DF5 2007-05-24
uid AG Projects Debian Package Signing Key <support@ag-projects.com>
sub 2048g/9565A2F3 2007-05-24
pub 2048R/9CA27E41 2018-02-02
uid John ATEL (Newkey) <info@newconcepttechnologies.com>
sub 2048R/36CFB265 2018-02-02
sub 2048R/9934ADF8 2018-02-02
/etc/apt/trusted.gpg.d/nilarimogard_ubuntu_webupd8.gpg
------------------------------------------------------
pub 1024R/4C9D234C 2010-01-20
uid Launchpad webupd8

Your system is looking for this key of 16D8F9F5. It is the last 8 numbers of the whole string. According to the web site of the ppa that is looking for that key http://ag-projects.com/news/new-openxcap-release-107/ all you should have to do is to run the following line to install the key:

sudo wget -O /etc/apt/trusted.gpg.d/agp-debian-key.gpg http://download.ag-projects.com/agp-debian-key.gpg

Just as a test I ran it and I see that key now installed:

sudo apt-key list
/etc/apt/trusted.gpg.d/agp-debian-key.gpg
-----------------------------------------
pub 4096R/16D8F9F5 2016-10-04
uid AG Projects Debian Package Signing Key <support@ag-projects.com>
sub 4096R/9AE893BC 2016-10-04

Hope this helps!